CORDIS - EU research results

Towards design and processing of advanced, competitive thermal barrier coating systems

Final Report Summary - TOPPCOAT (Towards design and processing of advanced, competitive thermal barrier coating systems)

The ultimate aim of the TOPPCOAT project was to make significant improvements to Thermal barrier coating (TBC) systems by using advanced bonding concepts in combination with additional protective and functional coatings. The first objective was to use the developments to provide a significant improvement to state-of-the-art Air plasma-spraying (APS) and hence, provide a cost-effective alternative to Electron-beam physical vapour deposition (EB-PVD). The second objective was to combine these new concepts with new coating technologies to provide new, advanced material for thermal barrier systems with a capability exceeding the performance of EB-PVD coatings. In both cases, a major impact on TBC performance, manufacturing and maintenance costs and hence competitiveness of European aviation gas turbine manufacture was expected.

A new method which produced a controlled, Three-dimensional (3D) surface morphology was used to both improve bonding and, crucially, enable control of the TBC microstructure to provide in the latter case a much higher segmentation crack density. In order to take full advantage of this, the use of advanced spraying techniques both to initiate the segmentation cracks and to control the coating process in order to obtain desired reproducibility was essential. This included online particle diagnostic systems, precise substrate temperature control and high substrate temperatures and the use of advanced APS gun technology. All were essential for control segmentation crack formation. The project was aimed towards the next generation of TBC systems. In addition to the techniques developed above, further innovate steps were investigated. There were new, emerging technologies which showed the potential to produce highly strain-tolerant coatings in a cost-effective way. The processes that were identified as having the most potential were Thin film-low pressure plasma spraying (TF-LPPS), Plasma enhanced chemical vapour deposition (PE-CVD), Nano-phase suspension plasma spraying and Hollow cathode gas sputtering PVD (GS-PVD). Investigations determined the single most promising process for the deposition of TBC systems. Also new, advanced TBC materials such as aluminates and modified spinels were included.

The development of more strain tolerant coatings with a performance at least equal to EB-PVD coatings was successful and brought out some results that may be the base for further investigations and qualifications. Highly segmented coatings produced via advanced APS processes showed some outstanding test results, while only the thermal conductivity in the same range as that of EB-PVD coatings was a disadvantage. The second innovative TBC system was the LPPS-TF coating, which constituted also a potential coating system to replace the expensive EB-PVD process. Another remarkable achievement was the further development of APS sensor coatings. Besides the already known use of the TBC sensor as a temperature indicator, an ageing detector or an early delamination sensor, another functionality was proven throughout the TOPPCOAT project: thickness measurement capability. Moreover, the sensor coatings showed remarkable durability during the cyclic oxidation tests which were comparable with the best coating morphologies tested during the project.

Moreover, the performance of suspension plasma sprayed coatings was convincing especially in the long-term furnace cycling test. The evaluation of different new TBC systems on real parts could not be finalised within the project, but further qualification tests would show whether highly segmented coatings or LPPS-TF coatings were applicable in modern gas turbines. LPPS-TF TBCs as repair technology for EB-PVD coatings offered a promising solution for lower repair costs of turbine parts. Furthermore, the engine test of a new developed bondcoat showed that costs couldn be reduced by using a High velocity oxy-fuel coating spraying (HVOF) bondcoat instead of state-of-the-art Vacuum plasma spray (VPS) bondcoats.